Lead (Pb) is widely recognized as a toxic substance, and the health and environmental issues related to the use of lead have been well documented over many decades. Lead poisoning is a serious health threat which usually occurs after a prolong exposure to lead and lead compounds. As a result, in the United States, the use of lead and lead compounds has already been banned from many consumer products. For example, tetraethyl-lead was formerly used as an “anti-knock” additive in gasoline, lead solder was used in plumbing applications, and of course in past decades lead was commonly found in paint.
Despite the concerted efforts of a number of different industries to eliminate the use of lead, it is still found in many consumer goods, including storage batteries, ammunition and electronic products. While storage batteries account for approximately 80% of lead consumption, its recycling program is very effective and therefore raises few health concerns. However, lead solder usage in electronic materials is of particular concern because these devices are rarely recycled resulting in the contamination of landfills when the products are discarded. More alarmingly, once in the landfill lead solder from electronic circuit boards can leach into the ground water system and also contaminate the soil.
Many countries around the world have taken steps to eliminate lead contamination from electronic products over the past two decades. The global electronic industry is under a lot of pressure from the European Union (EU) to completely phase out lead from many electronic products. Specifically, the EU enacted the directive called ROHS (Official Journal of the European Union, L37 19-23, Feb. 13, 2003,), or the Restriction of Hazardous Substances, which as of Jul. 1, 2006, banned lead (and a few other hazardous substances) from most consumer electronic products. Many countries in Asia including China, Korea, and Japan have come up with their own version of ROHS legislation. In the US, California's SB20 prohibits the sale of electronic devices which are prohibited under the EU's ROHS after Jan. 1, 2007.
As a result, there has been an ongoing research effort to find a substitute for Pb solder, but, until now, there has been no clear solution to the problem. In the current transition stage, the commercial Pb-free solders for reflow application in electronics packaging include a few varieties of near ternary eutectic of tin (Sn), silver (Ag) and/or copper (Cu) alloys with possible minute additions of elements such as bismuth (Bi), indium (In), zinc (Zn), and antimony (Sb). However, these Sn—Ag—Cu (SAC) and Sn—Ag-Bi (SAB) solders are only band-aid solutions to comply with ROHS. SAC solders are inferior to Pb-Sn solder in terms of solderability (wetting, spreading and low melting) and reliability. Each of these technical drawbacks can limit the effectiveness and applicability of these materials. For example, higher processing temperatures create a serious problem in a system with multiple joining processes, such as flip-chip packaging. The temperature of the last reflow process dictates the temperature of prior reflow processes. Specifically, in the case of electronics, replacing the traditional Pb-Sn solder with Sn—Ag raises the soldering temperature from 180° C. to 215-250° C. This in turn elevates the required melting temperature of prior reflow processes to above the 300° C. range to avoid subsequent remelting. Unfortunately, there are only a limited number of solders that satisfy these conditions. Moreover, there are other potential problems regarding the stability of substrates and other features on the chip that are not designed to withstand processing at these higher temperatures.
Likewise, molten lead has a very low surface tension, which contributes to its excellent wettability and spreading. Indeed, it has long been observed that the wetting characteristic of Pb/Sn solder far exceeds those of lead-free alternatives. At the interconnect interface, Pb/Sn solder forms chemical bond by creating a stable pure Sn compound. The replacement SAC solders would have three competing phases competing: β—Sn, Ag3Sn and Cu6Sn5. The two latter phases are non-equilibrium intermetallic compounds, which nucleate and grow with minimal undercooling. Adequate undercoating usually translates to the reduction of residual stresses. There are numerous studies showing the poor mechanical, thermal and electrical reliability of these two intermetallics.
Specifically, mechanical connection, electrical conduction and thermal pathway are three main functions of solder joints, particularly in the electronics industry. Mechanical attachment problems usually stem from the mismatch of the coefficients of thermal expansion between the materials attached to both ends of the solder joints and mismatch between the solder joint and attached substrate materials. During thermal cycling, the joint experiences shear stresses. Pb-solder joints release thermally induced stress by plastic deformation, which is not possible with SAC solder. For example, FIG. 1 provides a micrograph of a failed SAC solder joint after subjected to temperature cycling. Another shortcoming of SAC solder is electromigration with operation at high current density, as shown in FIG. 2. Because of the flaws of most of the current viable Pb-free solders, a direct and suitable replacement for traditional Pb-Sn soldering has not been found.